Skeletal muscle hierarchy. Germann & Stanfield, fig 13.1

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1 Skeletal muscle hierarchy Germann & Stanfield, fig 13.1 Muscle Fascicle Fibre (cell) Myofibril Sarcomere 1

2 Skeletal muscle fibres Enclosed by sarcolemma Rippled with caveolae at rest (invaginations of sarcolemma) Allow lengthening without membrane damage Surrounded by basement membrane Surround one, sometimes more than one fibre Loose glycoprotein & collagen network Provides framework for regeneration Quadriceps muscle group 1 million fibres per cross-section section Several million per whole muscle 2

3 Fibroblasts Reside between fibres Secrete thin collagen connective tissue matrix (endomysium) Perimysium - thick connective tissue layer Surrounds groups of fibres (fascicles) Contains blood vessels and motor axons Contains muscle spindles Epimysium thick outer connective tissue layer Surrounds whole muscle 3

4 Skeletal muscle structure Germann & Stanfield, fig

5 Skeletal muscle cross-section section Jones, Round & Haan,, fig 1.11 Ep epimysium; ; P perimysium; ; En endomysium; ; F - fascicle 5

6 Skeletal muscle myofibrils Average about 1μm 1 m in diameter myofilaments per myofibril Myofibrils are separated by: Sarcoplasmic reticulum network Transverse tubules Mitochondria (sometimes) Myofibrils per muscle fibre As little as 50 in foetal muscle As many as 2000 in adult muscle 6

7 The Sarcomere Germann & Stanfield, fig 13.3 Highly organised array of myofibrillar protein filaments μm m in length (Z-line( to Z-lineZ line) Contractile proteins (actin, myosin) Regulatory proteins (troponin, tropomyosin) 7

8 Histology Light Microscope Cross striations show A- A & I-bandsI Lateral cellular organelles Multi-nucleate Electron microscope Clear sarcomere structure A- & I-bandsI H-zone, Z-lines, Z M-linesM 8

9 Myofilament Arrangement Jones, Round & Haan,, fig filaments per myofibril Arrangement 1:6 for myosin (thick) 1:3 for actin (thin) Skeletal muscle fibre composition by volume 83% myofibrils myofilaments 9% sarcoplasmic reticulum 4% terminal cisternae 5% longitudinal tubules 1.5% mitochondria 0.3% t-tubulest tubules NB1: values taken from frog sartorius muscle NB2: values will vary with species and fibre type Mobley & Eisenberg. J Gen Physiol 1975;66:

10 Skeletal muscle organelles Transverse tubules (T-tubules) Invaginations of the cell membrane (sarcolemma) Penetrate deep into each muscle fibre Humans 2 per sarcomere evident at A-I A I junction Amphibians 3 per 2 sarcomeres evident at each Z-lineZ Function to evenly distribute action potentials Sarcoplasmic reticulum (Ca 2+ storage) Membrane network surrounding each myofibril 2 connected regions Terminal cisterna (Ca 2+ release) into cytosol Forms TRIAD with T-tubules T at Z-line/IZ line/i-band region Longitudinal tubules (Ca 2+ uptake) Located around myofibril A-bandsA 10

11 Skeletal muscle organelles Boron & Boulpaep,, fig

12 Muscle Organelles Jones, Round & Haan,, fig

13 Skeletal muscle TRIAD Electron micrograph Amphibian skeletal muscle Junctional area between: Transverse tubules Usually appears flattened at the triad Sarcoplasmic reticulum Terminal cisternae One portion flanking a single T-tubuleT Site of E-C E C coupling Excitation-Contractionontraction 13

14 Excitation-Contraction Coupling The process of action potential transduction to provide an intracellular signal ( [Ca( 2+ ] i ) and subsequent muscle cell contraction Most commonly focuses on the links between: T-tubule action potentials, and Sarcoplasmic reticulum Ca 2+ release Mechanism is not entirely understood Dulhunty (2006). Excitation-Contraction coupling from the 1950s into the new millennium. CEPP,, 33: Much is understood about the function of some required components Voltage sensor (Dihydropyridine( receptor DHP) Ca 2+ release mechanism (Ryanodine( receptor RyR) Structure, function and operation of the fast link between the twot 14

15 E-C C coupling Berne & Levy, Chapter 12 15

16 E-C C coupling Jones, Round & Haan,, fig

17 E-C C Coupling Jones, Round & Haan,, fig 3.5 DHP & RyR Tetrad proteins 4 subunits Precise alignment in opposing membranes Twice as many RyR s Redundant, or Provide signal amplification DHP (T-tubule voltage sensor) L-type Ca 2+ channel Little or no ion flux in skeletal muscle Flux in cardiac muscle: CICR (Ca 2+ -Induced Ca 2+ Release) RyR (SR Ca 2+ release channel) Sensitive to ligands Caffeine Ca 2+ (CICR) 17